Currencies Futures Options vis-a-vis equity options
Using end-of-day data, equity indices exhibit about double the amount of volatility than the major currency pairs. Take a look at these statistics. 9.5 years of data was used to compile the below - from the 3rd January, 2000 to 5th August 2009. calculated both 30 day and 100 day historical volatility and then averaged this across all data sets.
The 3 stock market indices averaged 26.26% while the 15 currency pairs averaged 12.14%. Given that currency volatility is, on average, almost half that of an equity index, you could assume that option premiums are relatively half as cheap also.
Here's the summary from optionstradingtips.com: